Biodiesel, such as soy diesel (methyl soyate), is becoming increasingly useful as a “green fuel”. Biodiesel is a biodegradable and nontoxic alternative to diesel fuel. It is made from renewable biological sources such as vegetable oils and animal fats (Bioresource Technology 1999, 70, 1-15). Biodiesel fatty acid methyl esters have been recently accepted as a viable alternative to traditional petroleum-derived solvents, which are of environmental concern and are under legislative pressure to be replaced by biodegradable substitutes. Although interest in biodiesel is rapidly increasing, the process by which biodiesel is synthesized has not changed much in recent years.
Currently, soy diesel (methyl soyate) is made commercially by an energy and labor-intensive process wherein soybean oil is reacted with methanol at 140-150° F. (about 60-65° C.), often under pressure, in the presence of sodium methoxide to yield fatty acid methyl esters and glycerol. This process is called “transesterification”. Isolation of the desired methyl soyate from the highly caustic (toxic) catalyst and other products, such as glycerol, involves a precise neutralization process with strong acids, such as hydrochloric acid (HCl), and extensive washes with water to remove the resulting sodium chloride (NaCl) salt. Also, the glycerol must be separated from the sodium chloride salt by vacuum distillation in an energy intensive operation for this high-boiling product (Bioresource Technology 1999, 70, 81; Fuel 1998, 77, 1297; J. Am. Oil Chem. Soc. 1985, 62, 331; J. Am. Oil Chem. Soc. 2001, 78, 139).
Researchers worldwide have been developing solid catalysts for the transesterification of oils to biodiesel. For example, various basic metal oxides, such as magnesium methoxide, calcium oxide, calcium alkoxide, and barium hydroxide (Applied Catalysis, A: General 2000, 192, (1), 23-28), have been demonstrated to be active catalysts for transesterification. However, the recyclability of these solid base catalysts is poor. This is because of the moderate solubility of some of these solid metal oxides and hydroxides in methanol (Bioresource Technology 1999, 70, (3), 249-253). Furthermore, these base catalysts are not suitable for feedstocks other than soybean oil, such as waste restaurant oils and rendered animal fats. The large amount (5-15 wt. %) of free fatty acids (FFAs) contained in these feedstocks significantly shortens the lifespan of base catalysts because of the saponification.
Currently, sulfuric acid, a homogeneous strong acid, is used as a pretreatment catalyst for converting FFAs to biodiesel. However, the need for neutralization before the transesterification reaction again creates economical and environmental concerns. While several solid acids, such as zeolite, ion-exchange resins, and sulfated zirconia, have been tested for FFA esterification (Advanced Synthesis & Catalysis 2006, 348, 75-81; Accounts of Chemical Research 2002, 35, (9), 791-797), it would be desirable to develop an integrated, acid-base cooperative system that can catalyze both esterification and transesterification reactions.
What is needed is a process for biodiesel production that does not require aqueous washes and neutralization steps, and a catalyst for that process that can be easily separated from the biodiesel products. An economical and recyclable catalyst for the conversion of oils to biodiesel is also needed. A catalyst that can economically catalyze both the esterification of free fatty acids and transesterify oils to biodiesel is further desired.